1. Field of the Invention
This invention generally relates to the field of electronic devices, and more specifically to thermal interposers, or heat spreaders, for thermal management and cooling of semiconductor devices.
2. Description of Related Art
Thermal management is of great importance to the operation of electronic devices. Thermal management is especially important in the operation of semiconductor devices as factors such as increasing operating frequencies push power consumption, and therefore heat generation, to the limits of the cooling capacity of traditional passive, air-cooled, heat sink technology. The power density (W/cm2) in semiconductor devices continues to increase as the circuit density and operating frequency increase. Thermal management includes the skill of dissipating the heat generated by an electronic device away from the device and allowing the generated heat to disperse to its surroundings, while maintaining the semiconductor device at as low a temperature as possible. Insufficient transfer of heat away from an electronic device can result in performance and reliability degradation of that device or circuit due to an unacceptably high operating temperature.
Typical thermal management solutions use some combination of aluminum or copper heat sinks, fans, thermal spreaders/heat pipes, and thermal pastes or adhesives (Thermal Interface Materials or “TIMS”) to form a low thermal resistance path between the semiconductor chip and the ambient. Typically, high performance semiconductor chips have one or more “hot-spots”, which are regions of the chip having a power density that is substantially greater than the average power density (e.g., two to three times the average power density). To insure reliable long term operation, the requirements for the thermal management solution must be driven by these hot spots, not just by the average power density of the chip.
One conventional technique for providing improved thermal management is the use of micro-channels. The structure utilizes a set of narrow channels that may be introduced into the integrated circuit chip itself or into a cooling member that is in intimate contact with the chip. In either case, the channels have heights and widths on the order of several tens of microns and are encapsulated by either the chip or the cooling member in intimate contact with the chip. This results in an array of micron-sized rectangular tubes through which a cooling liquid is forced so as to provide for dissipating the heat generated within the integrated circuit chip.
Another conventional technique for providing improved thermal management is the use of a vapor chamber as a heat spreader. A vapor chamber transports heat within a system with a low thermal resistance. FIG. 9 illustrates a cross sectional diagram of a conventional thermal management structure that includes a vapor chamber. As shown, the front surface of the semiconductor chip 12 is electrically connected to an organic package 14 using micro solder balls 16 (e.g., C4's). The chip 12 is sometimes underfilled with a polymer (not shown) for improved reliability. The packaged chip is then mounted onto a printed circuit board 18 using larger solder balls 20 (e.g., a BGA or ball grid array). The primary thermal path for the chip 12 is from the back surface of the chip, through a layer 21 of a first thermal interface material (TIM), which is typically a compliant grease or paste that is filled with particles of a thermally conductive material (though a thermally conductive compliant adhesive such as an Ag filled silicone can also be used), through a vapor chamber thermal spreader 22, and then through a second TIM layer 23 to an air cooler heat sink 24.
In order to achieve an acceptable operating temperature for the chip, it is necessary to minimize the total thermal resistance (° C./W) from the chip to the ambient. The TIM layers provide mechanical compliance to relieve the thermal expansion mismatch stress between the components that are constructed of different materials, but they also represent a significant portion of the total thermal resistance. The vapor chamber 22 laterally spreads the heat so as to reduce the power density (W/cm2) to reduce the total temperature drop. Because they can be fabricated from similar materials, the vapor chamber heat spreader 22 can be integrated with the heat sink 24 into one unit.
The vapor chamber 22 is a vacuum vessel having a wick (or groove) structure lining its inside walls. The wick structure is saturated with a working liquid. As heat is applied during operation, the fluid at that location in the wick structure is vaporized and the vapor rushes to fill the vacuum. Wherever this vapor comes into contact with a cooler wall surface, it condenses and releases its latent heat of vaporization. The condensed fluid returns to the heat source via capillary action, and is then ready to be vaporized again to repeat the cycle.
For example, in the conventional structure of FIG. 9, vaporization primarily occurs in the region of the bottom surface of the vapor chamber that is above the chip and condensation primarily occurs on the top surface of the vapor chamber that is attached to the heat sink. The condensed working fluid is transported to the perimeter of the vapor chamber and then is transported from the wick structure on the top inner surface, down the side walls, and to the wick structure on the bottom inner surface. The capillary action of the wick that lines the inside walls enables the vapor chamber to work in any orientation with respect to gravity. However, such a conventional vapor chamber is typically fabricated out of metal. Thus, it cannot be rigidly joined to the integrated circuit chip due to a thermal expansion mismatch.
Yet another conventional technique for providing improved thermal management is the use of heat pipes. A heat pipe operates similarly to a vapor chamber, but a heat pipe is designed to transport heat from the heat source to a remote heat sink. A heat pipe is typically used when there is not sufficient space available directly over a chip to mount a heat sink so that the heat sink must be mounted at a remote location. A typical heat pipe for semiconductor devices is a circular metal tube with a wick structure on the inside. A portion of the heat pipe is filled with a working fluid and the pipe is sealed. The heat pipe transfers energy through the evaporation and condensation of the fluid. More specifically, as heat is applied to the evaporator region of the heat pipe, the fluid in the channel vaporizes, and thus removes the heat. The vapor travels through the channel to the condenser region of the structure, where the heat sink is attached to the heat pipe and the heat is released during condensation. The working fluid is then transported back to the evaporator region along the wick structure in the pipe by the capillary effect. As with a conventional vapor chamber structure, because the heat pipe is fabricated from a metal (such as copper), it cannot be rigidly joined to an integrated circuit chip due to a thermal expansion mismatch. Instead, a TIM layer is needed to provide mechanical compliance. However, as explained above, a TIM layer has a larger thermal resistance than is desired.
There have been a number of efforts to provide improved vapor chamber heat sinks. For example, in U.S. Pat. No. 4,327,399, Sasaki et al. describe a heat pipe structure that includes a wiring substrate of ceramic or silicon having a cavity in the interior. On the inner surface of the cavity there is formed a wick. Radiating fins are formed at the end of the substrate and chips are securely inserted in holes that communicate with the cavity so that the wick on their upper surfaces is aligned with the wick on the inner surface of the cavity for directly cooling the integrated circuit elements. However, this structure is very complicated and presents practical disadvantages such as difficulty in electrically connecting and reworking the chips.
Others efforts have proposed forming a vapor chamber heat spreader from silicon so that it can be directly joined to the back of a chip. Such silicon vapor chambers have been described by D. S. Shen et al. in “Micro Heat Spreader Enhanced Heat Transfer in MCMs” (Proceedings of the 1995 IEEE Multi-Chip Module Conference; MCMC '95, Santa Cruz, pp. 189–194), D. A. Benson et al. in “Micro-Machined Heat Pipes in Silicon MCM Substrates” (Proceedings of the 1996 IEEE Multi-Chip Module Conference, MCMC '96, Santa Cruz, pp. 127–129), C. Gillot et al. in “Silicon Heat Pipes used as Thermal Spreaders” (Proceedings of the 2002 IEEE Inter Society Conference on Thermal Phenomena, pp. 1052–1057), and C. Gillot et al. in “Silicon Heat Pipes used as Thermal Spreaders” (IEEE Transactions on Components and Packaging Technologies, Vol. 26, No. 2, pp. 332–339).
Shen et al. and Benson et al., and U.S. Pat. Nos. 5,769,154 and 6,056,044 describe wick structures formed in silicon along with heat pipes formed by bonding a silicon substrate containing a wick structure to a cap containing vapor channels. However, a disadvantage of such a structure is that the wick is only provided on one surface of the interior cavity of the vapor chamber. The publications by Gillot et al. describe silicon heat pipes that are formed by bonding together a stack of three wafers, with large portions of the middle wafer being removed to form vapor passage cavities. Fine channels are formed on the inside surfaces of the top and bottom silicon wafers as wick structures. However, such a structure is complicated and its performance is limited because the fluid return is only around the perimeter region. Additionally, the use of secondary wick structure for conventional metallic heat pipes has been proposed by J. Zuo in U.S. Patent Application Publication No. 2002/0139516 A1. However, such a structure would be difficult to implement using either of the silicon vapor chamber structures described above. Further, metallic vapor chambers present the disadvantages described above.
Therefore, a need exists to overcome the problems with conventional thermal management techniques as discussed above, and particularly for an improved vapor chamber heat spreader that can be attached to the back surface of a semiconductor device so as to give a very low thermal resistance, especially for areas of the semiconductor device that contain local hot spots.